Conventional apparatus for automatically mounting electronic components on the surface of a printed wiring board each in position comprise a suction head mechanism having a suction nozzle for attracting the electronic component by suction. The suction head mechanism is mounted on a reciprocating device which is movable as controlled in the directions of X-axis, Y-axis and Z-axis.
When the above apparatus is used for surface mounting, the suction head mechanism is first moved to a component feed portion of an electronic component feeder, the suction nozzle is caused to attract an electronic component thereto by a vacuum, and the component is thereafter moved to a position above a printed board, whereupon the suction head mechanism is lowered to place the component in position on the surface of the board.
Upon the component coming into contact with the printed board, the suction head mechanism is further driven downward, whereby the component is pressed against a bonding layer or solder paste layer on the board surface and fixed or temporarily fixed to the board.
The suction head mechanism is thereafter returned to above the component feeder, followed by the subsequent cycle of surface mounting operation.
With the conventional surface mounting apparatus, the upward and downward movement of the suction head mechanism is effected by operating a cam mechanism. The cam mechanism has a cam curve which is so designed that an excessive impact force or pressure will not act on electronic components when the suction nozzle is pressed against the component on the feeder and also when the attracted component is pressed against the printed board.
In recent years, however, a wide variety of electronic components are introduced into use which differ variously in configuration, size (thickness) and material. These components include, for example, delicate parts such as IC (LSI) bear chips. The electronic components to be mounted by the surface mounting apparatus therefore differ greatly in mechanical properties with the type of components.
Accordingly, in mounting different types of electronic components by the surface mounting apparatus, the pressure to be exerted on the component when the component is held by suction and when it is mounted on the surface must be controlled to a suitable value for each of the different types.
The suction head mechanisms already proposed for use in surface mounting apparatus include, those which are driven by a linear motor (Unexamined Japanese Patent Publications SHO 60-66500 and SHO 63-232496), those which are driven by a compression spring (Unexamined Japanese Patent Publication SHO 63-22292), those wherein electropneumatic regulator is utilized (Unexamined Japanese Patent Publication HEI 1-246899), etc.
However, the apparatus wherein the linear motor is used for driving has the problem that the apparatus is complex in construction and has a great weight. With the apparatus wherein the compression spring or electropneumatic regulator is used, it is difficult to accurately control the pressure to be actually exerted on the electronic component.
To overcome this problem, a system appears useful which comprises a piezoelectric element mounted on the component suction head for measuring the pressure to control the upward and downward movement of the head in accordance with the output voltage of the piezoelectric element and to press the electronic component against the printed board with a predetermined pressure.
Electrically, the piezoelectric element can be regarded as a capacitor. When subjected to a force, the piezoelectric element produces a charge in proportion to the acting force as expressed by Equation (1). EQU Q=d.sub.33 .multidot.F (1)
wherein Q is the amount of charge (C), F is the acting force (N), and d.sub.33 is an equivalent piezoelectric constant (.times.10.sup.-12 C/N).
Accordingly, the pressure can be measured by detecting the charge produced from the piezoelectric element.
On the other hand, a charge amplifier as shown in FIG. 2 is generally used for detecting the charge produced by the piezoelectric element. The illustrated charge amplifier 7 comprises an OP amplifier 71 of the FET input type connected to an output terminal of a pierzoelectric element 61, and a resistor Rf and a capacitor Cf included in a feedback loop of the amplifier. The charge produced by the element 61 is converted to a voltage signal and output by the amplifier 7.
The resistor Rf connected in parallel with the capacitor Cf of the feedback loop serves to stabilize the dc component.
The output voltage of the charge amplifier 7 attenuates in accordance with a time constant which is dependent on Cf.multidot.Rf (curve a-c shown in FIG. 3). The attenuation can be diminished by increasing the values of Cf and Rf.
However, the value of Cf can not be increased without a limit to assure the charge amplifier 7 of a gain since the gain is the reciprocal of the Cf value. The value of Rf for giving a stabilized dc component can not be increased limitlessly, either.
Further at the time point of completion of a pressing action for mounting an electronic component, the capacitor Cf is charged in the negative direction by an amount corresponding to the charge which leaked toward the resistor side upon pressing, so that the output voltage at that time point lowers from point c to point e as shown in a broken line in FIG. 3 and thereafter gradually increases toward the positive direction.
Accordingly, if another pressing action is performed at a time point at which the output voltage has yet to return to zero level, the relationship between the output voltage and the pressure deviates by an amount corresponding to the negative output voltage at the time point to make accurate measurement impossible. This problem becomes pronounced owing to a cumulative effect in the case where Cf and Rf are each set at a great value.
Not only when the charge amplifier is used but also when an OP amplifier of the FET input type is used as the detection circuit to which the piezoelectric element is to be connected, the capacitance of the piezoelectric element and the resistance of the detection circuit provides a time constant, so that a similarly problem is encounted also in the latter case.